Guest Toilet Etiquette..?!

I have two regular guests who do not have much in the way of manners when it comes to staying at someone's home for a few days.. There are a number of issues but the one I am concerned with today is their toilet etiquette.

Every time they leave us after a few days' stay, their ensuite toilet is left in a state which is, let's say, 'well used'. I then have the unenviable task of clearing up the mess inside the toilet bowl and this makes me feel at times disgusted at other times completely furious! They have been doing this for about nine years, ever since they had an ensuite joined to the room in which they stay. It feels like some sort of pronouncement on the quality of their stay, as if we have not looked after them enough and they are saying in a nasty way that our performance wasn't up to scratch. Perhaps a sort of 'this is what we thought of our stay' or 'here's something to clean up'. It makes me so cross!

I have nearly plucked up the courage to ask the female guest (politely, quietly, away from the others so as to minimize any subsequent embarrassment) to check the toilet before leaving us but this morning I mentioned my intention to my husband and he asked me not to do so in case it spoiled the last bit of time they had with us before their onward holiday...

What on earth should I do?! I thought about making a pretty sign and putting it in a frame on the wall, saying 'please leave as you would wish to find' or similar... But isn't that just too ridiculous after nine years of them soiling and leaving? Is it a case of me being overly fussy and I just need to put up and shut up? Should I just pay a cleaner to come in and sort it out once they've left (actually I'd be too embarrassed about the mess and probably end up cleaning it beforehand anyway!!)

If it was up to me I would never want these guests to return (I've been feeling like that for years!) but they are really guests of my husband and so it is not my place to refuse them. My husband has offered to clear the mess but I haven't the heart to make him!

Sorry a lot of fuss over nothing (?) just feeling a bit fed up this morning!

Any suggestions on how to deal with this with minimal fuss, but without having to get my hands dirty, so to speak!

Thank you!